1. Field of the Invention
The present invention relates to an electrode of a semiconductor device and, more particularly, to a photoelectric converting device having a collector electrode which causes lesser deterioration with time, has a low resistance, and can be made thick.
2. Related Background Art
A photoelectric converting device is a semiconductor device known as a photosensor, a photoelectric cell, or a solar cell, which is an energy generating device using the photovoltaic power of a semiconductor.
To alleviate an energy crisis caused by an insufficient energy supply power due to a recent abruptly increasing demand for energy or to mitigate a global environmental disruption by a warming phenomenon resulting from a greenhouse effect caused by an increase in carbon dioxide quantity, an energy supply source which is less harmful to environment has been desired.
In particular, a photoelectric converting device (which will include a photovoltaic device and a solar cell hereinafter) which is clean, has a high safety, and can supply energy permanently for extended periods of time is considered most prominent as a novel energy supply source that can meet the above requirement.
Well-known examples of the photovoltaic device are as follows.
(1) Crystal-based solar cell
A crystal-based solar cell is manufactured by forming a p-n junction by doping an n-type or p-type impurity into a p-type or n-type single-crystal or polycrystalline wafer.
(2) Amorphous silicon-based solar cell
An amorphous silicon-based solar cell is manufactured by decomposing monosilane gas or disilane gas by using, e.g., heat, an RF electric field, or light, thereby producing and depositing a-Si or the like.
In this case, an amorphous silicon-based solar cell with a pin structure is formed by forming a p-type semiconductor and an n-type semiconductor by doping a gas, such as B.sub.2 H.sub.6 or BF.sub.3, as a p-type dopant and PH.sub.3 as an n-type dopant simultaneously with monosilane or disilane.
(3) Compound semiconductor solar cell
A compound semiconductor solar cell is a GaAs solar cell manufactured by forming a p-n structure by growing p-type GaAs on n-type GeAs by liquid-phase epitaxial growth.
A solar cell in which a p-n structure is formed by stacking and calcining n-type CdS and p-type CdTe is also a compound semiconductor solar cell.
In addition to the above solar cells, a CuInSe.sub.2 solar cell and an n-type CdS/p-type CuInS.sub.2 solar cell are usable as photovoltaic devices. These photovoltaic devices aim at efficiently extracting an incident optical energy as an electrical energy through a collector electrode.
To obtain a photovoltaic device with a high conversion efficiency, therefore, the following two points are important.
(1) To increase the efficiency of conversion from an optical energy to an electrical energy.
(2) To form a collector electrode with a lower electrical resistance for the purposes of decreasing a Joule loss by decreasing the electrical resistance at the collector electrode in collecting an electrical energy through the collector electrode and of reducing blocking of light by the collector electrode.
The following two methods are known as a method of forming a collector electrode.
One is a method of forming a collector electrode by vapor-depositing a conductive metal, as disclosed in Japanese Patent Publication No. 57-5066.
The other is a method of forming a collector electrode by printing and calcining a conductive paste prepared by binding a conductive powder using glass or a polymer.
In particular, the method using a conductive paste as a collector electrode is one of techniques which can easily form large-area devices through a simple manufacturing process and are therefore expected to have a surprising effect of reducing the manufacturing cost.
At the present, however, collector electrodes manufactured by these conventional methods have a problem of a decrease in conversion efficiency or an inability to sufficiently achieve a stable operation over long periods of time resulting from the following respects.
(1) The electrical resistance of a collector electrode itself is relatively high, leading to an increase in Joule loss, and this increases the loss of a conversion efficiency.
(2) Since a collector electrode oxidizes with a long-term use, its electrical resistance further rises from the initial electrical resistance, increasing the loss of a conversion efficiency.
(3) A collector electrode peels from a semiconductor or a transparent electrode under the influence of humidity to gradually degrade the function as a collector electrode.
(4) Some material, such as a polymer binder, optically degrades upon irradiation with light to produce fine cracks or cause breaking of a collector electrode, causing the collector electrode to lose its function.
In these respects, some improvements have been made to decrease the electrical resistance of a collector electrode or to stabilize the operation of a collector electrode over a long time period. One example is a method of binding conductive powders of gold and silver with low-melting glass, as disclosed in Japanese Patent Publication No. 61-59549.
Other examples are a method of binding gold, silver, and aluminum with low-melting glass, as disclosed in Japanese Patent Publication. No. 61-59551 (U.S. Pat. No. 4,256,513) and a method of binding a conductive powder consisting of a nickel-antimony alloy by using a binder, as disclosed in Japanese Patent Publication No. 62-8958 (U.S. Pat. No. 4,342,796).
Japanese Patent Publication Nos. 63-11791, 2-3553, 2-3554 (U.S. Pat. No. 4,486,232), or 2-3555 also discloses a method of adding various metals, such as bismuth and a rare earth element, to silver and binding the resultant material by using a binder.
In any of the above methods, however, the volume resistivity of a collector electrode is approximately 30 to 50 .mu..OMEGA..multidot.cm, i.e., is not sufficiently lowered as that of a collector electrode of a photovoltaic device, and this increases the loss of a conversion efficiency. In addition, no satisfactory solutions have been obtained yet in long-term stability, humidity resistance, and light resistance.
As shown in FIG. 9, a conventional collector electrode of a photovoltaic device is often fabricated by printing a conductive paste material obtained by monodispersing fine silver particles about 1 to 10 .mu.m in diameter in a polyester-based, polyimide-based, epoxy-based, or phenol-based thermosetting resin or in a glass frit, and mixing the dispersant with an organic solvent, such as Cellosolve, for controlling the viscosity on a transparent electrode layer 71 as an electrode formation surface of a photovoltaic device by using a stencil screen or the like, and thermally hardening the resultant structure. This structure makes it possible to form an electrode at a high productivity and a high material yield by using a material with a very large area, largely contributing to a reduction in the manufacturing cost that is currently one major problem of photovoltaic devices.
In this case, as a collector electrode 74 made from the conductive paste, one having a line width d of 100 to 150 .mu.m and a thickness h of 10 to 20 .mu.m is common for the mass-production purpose.
Various substances can be used as the material of the conductive paste. In an amorphous silicon-based solar cell which is apt to be easily damaged by high-temperature treatments, however, it is common to use a polymer paste prepared by monodispersing fine copper or silver particles about 1 to 5 .mu.m in diameter in a polyester-based, polyimide-based, epoxy-based, or phenol-based thermosetting resin.
Recently, however, with improving photoelectric conversion efficiencies or increasing areas of photovoltaic devices, a demand has arisen for a collector electrode with a smaller loss than those of the conventional collector electrodes, and so collector electrodes formed using only a conductive paste have become unable to meet these requirements. As an example, the collector electrode 74 formed from the conductive paste consisting of the thermosetting resin has a specific resistance of 3.times.10.sup.-5 to 5.times.10.sup.-5 .OMEGA..multidot.cm. In addition, when fine printing with a smaller line width is performed in order to reduce a loss produced by blocking of light by the collector electrode, a printing variation increases or the film thickness decreases accordingly, and a generation loss derived from the resistance of the collector electrode largely increases. Furthermore, heating for a long time period is required to harden such a conductive paste, so it is practically difficult to obtain a thick electrode by coating layers of the paste one atop the next.
As shown in FIG. 10, therefore, a collector electrode according to the present invention has a structure in which a material 87 with a low specific resistance, such as solder, is placed on a conductive paste 84 as described above. This collector electrode causes lesser deterioration with time and has a low specific resistance.
In addition, it is desirable to increase the film thickness in order to improve the efficiency of a collecting function. If, however, a material with a high wettability with respect to solder is simply used as a conductive paste to increase bonding properties with respect to the solder, the curvature of the solder material decreases and no sufficient film thickness can be obtained.
Methods using vapor deposition or sputtering are also known as the electrode formation method. These methods, however, are almost never used in the manufacture of large-area devices because the manufacturing cost increases.
Note that losses produced by a collector electrode in a photovoltaic device are roughly classified into a loss caused by blocking of light by an electrode material and a resistance loss derived from a resistance that the electrode material has. To reduce the loss resulting from blocking of light, the line width of an electrode must be decreased.
When, however, the line width is decreased in order to reduce the loss caused by blocking of light, it becomes difficult to obtain a large thickness in screen printing, and the resistance increases compared to the decrease in the line width under the influence of printing variations.
For example, the collector electrode formed by the conductive paste consisting of a thermosetting resin mentioned earlier in the conventional example has a volume resistivity of 3.times.10.sup.-5 to 5.times.10.sup.-5 .OMEGA..multidot.cm. In addition, when fine printing,is performed to reduce the loss derived from blocking of light by the electrode, a printing variation increases or the film thickness decreases accordingly, resulting in a large increase in the resistance loss of the collector electrode. Furthermore, since a conductive paste of the above sort must be heated for a long period of time to be hardened, it is difficult to obtain a large thickness by coating layers of the paste one on top of another.
FIG. 29 shows the structure of a collector electrode in which a material with a low volume resistivity is placed on a bound conductive member consisting of the conductive paste described above.
FIGS. 28A to 28F illustrate a practical example of the steps of fabricating this structure. First, a conductive paste 20202 of the above sort is printed on a transparent electrode (SnO.sub.2, InO.sub.3, or ITO) 20201 of a photovoltaic device (FIG. 28A), and the resultant structure is thermally hardened in a thermodrying oven (FIG. 28B).
It was found, however, that when a solder layer 20203 was formed on the conductive paste 20202 by using molten solder, this solder layer 20203 was very thin and hence unsatisfactory to improve the resistance loss although the layer had an effect of increasing an environmental resistance. FIG. 27 is a graph showing the thickness of a collector electrode with solder formed when molten solder was dip-brazed onto a bound conductive member consisting of a polymer type conductive paste.
The present inventor, therefore, printed a solder paste, formed by mixing fine solder particles 30 to 50 .mu.m in diameter with a cream flux, by using a metal plate, and thermally melting the resultant structure in a hot-air drying oven. As a result, a thick electrode with an average film thickness of 30 .mu.m for a width of 300 .mu.m was realized, indicating a considerable improvement compared to the electrode formed by the dip-brazing.
If, however, the amount of a solder paste to be placed is simply increased by increasing the film thickness of a metal plate in order to realize a larger thickness than that obtained by the above process, the solder on the bound conductive member sometimes reaches a saturated critical amount. In such a case, the solder concentrates on a particular portion of the bound conductive member, forming a giant solder ball.